Here's a guide for picking bettas for show. I'm including video and pictures :)

Okay so in most cases you can't just pick a fish from a local pet store and end up with a show quality betta. Most show bettas come from several generations of hard work by their breeder. If you are considering showing, you already know this.

So let's start with some basic fin and form ideals. Learn more about each and every tail type you plan on showing.

Fins:
Symmetry is very important! Uneven fins are not good. For a halfmoon you want to see fins overlapping. Like this male. All show bettas must have a 180 spread.

Body:
Body type is often overlooked. A good betta will have a long, thick, torpedo shaped body. A scrawny, wimpy looking betta is not a good show fish. You want a lot of thickness BUT you don't want a short body. Always go for a longer bodied fish with a smooth top line. Here's some examples.

Here's a good video on determining the quality of your fry at a young age.

Here are a few of my personal choices for determining a quality show betta.

You cannot get much better than this for a Doubletail Halfmoon caudal. Nice even lobes that have a strong 180 spread that are clean cut. This photo was taken when he was younger but his finnage has grown in a little more and now they all lay together very nicely to make a arc without any bumps or edges. On a double tail it is common to have a short, stocky body so make sure you breed them to a girl who is long backed.

Good even symmetry throughout all fins and a strong dorsal base which is half the length of his topline. Notice the thick body and smooth body shape.

This is a new boy I will be receiving soon, I just thought I would put this right next to his picture so you can see the similarities.

A few things would need to be tidied up such as strengthening the first ray of the dorsal so it is longer and then sharpen the anal point a smidgen more. But he has the ideal caudal, body shape and thickness and his overall symmetry for a asymmetrical HMPK is near perfect.

The girl he will be bred to.

I thought this would be good to put in so you not only see a good show specimen, but know what to get for the partner in order to compliment the stock you have. This girl has an awesome dorsal, wide base and shape with the rays I need to make the dorsal point forward with a longer ray. This picture does not show it well but her caudal is a strong D shape I would prefer sharpers edges, that is where the boy's caudal comes in. Her anal is level and has a good shape to it. Her topline and body thickness could be improved, once again why the boy compliments her like a glove.

There's a lot of ways to go about quality and hard to get a sample of a perfectly formed fish.
Like Sincerely mentioned, the idea is to pick a pair that will compliment eachother in terms of bad and good points and breed to better their form.
On the fish above shown I can see some weak points but mostly strong ones... you must train your eye to find faults... if you don't find them or are willing to do selective breeding to improve them, then you know you got yourself a good candidate. Don't get blindsided by a fish's great coloring and big fins... it happens a lot.
I'm very picky. Why? Because I find it hard enough work to raise a spawn. I don't want to spend my time and $$ on trying to re-invent the wheel. I want my fish to be of quality so I start with the best fish I can get a hold of.
I think color is another thing some new breeders don't pay attention to. If you are breeding for show, your fish should be able to fall in a color/fin cathegory. Mixing random color patterns is not good as your fry might not have good color definition.
And if you need help... then ask! We are more happy to help you pick the perfect fish for your breeding goals!

It can be easy to find a good breeding candidate, then finding another fish to compliment the one you have can be the difficult part. Very true about the coloring, as you can tell from the two HMPK I posted they are both Blue/Yellow bi-colors with metallic bodies. I could work with a marble but then I would loose that bi-coloring to marbles. So it is just as important in breeding show fish to find colors/patterns that compliment!